In data communication using the Internet, etc., when an internal network, such as a specific carrier's data communication network or a corporate intranet, is connected to the Internet, in order to ensure high-speed access and the safety of communication, a proxy server is conventionally used. For such a proxy server, an HTTP (Hypertext Transfer Protocol) proxy, an FTP (File Transfer Protocol) proxy, etc., where an application layer protocol usable between a terminal and its communication destination is limited are widely used.
On the other hand, a data communication device is known that implements data communication using a plurality of application layer protocols (see the following Patent Document 1). The data communication device has the function of allowing, when receiving a SIP (Session Initiation Protocol) message from a client that includes protocol information, the client to perform data communication by a corresponding protocol.